This invention pertains to an inkjet ink, in particular to an aqueous inkjet ink comprising self-dispersible pigments and selected polyurethanes ink additives and to methods of using these in inkjet inks.
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Traditionally, pigments were stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. More recently “self-dispersible” or “self-dispersed” pigments (hereafter “self-dispersed pigment”) have been developed. Self-dispersed pigments are dispersible in water without dispersants.
Self-dispersed pigment is often advantageous over traditional dispersant stabilized pigments due to greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink.
Prints made with self-dispersed pigment ink, however, tend to be susceptible to rub off and poor highlighter resistance. The combination of self-dispersed pigment and dispersant stabilized pigment to improve image properties is taught in EP 1158030 and discloses the use of a CABOJET™ 200 results in poor performance to highlighter resistance. In EP 1114851, a sulfonated C.I. Pigment Red 122 is disclosed to have poor rubbing/scratch resistance when a highlighter pen was rubbed over the printed characters.
Polyurethanes have been described as ink additives in U.S. Pat. No. 7,176,248 and U.S. Patent Application Publication No. 2005/0176848. However, neither describes the combination of self-dispersed pigment and the polyurethanes derived from polyether diols where there is an excess of isocyanate reactive groups.
In U.S. Pat. No. 7,348,368 polyurethanes are described for use as additives to ink jet inks and these are used with polymerically dispersed pigments and acid numbers of greater than 50. In U.S. Patent Application Publication No. 2008/0207811 polyurethanes are described with acid numbers above 60 and have extensive examples with polymerically dispersed pigments. However, none of these describes the combination of self-dispersed pigment and the hydroxyl terminated polyurethanes with acid numbers below 50 which are derived from polyether diols that have at least 3 carbons atoms in the repeating ether group.
While inks based on aqueous dispersions with polyurethane additives have provided improved ink jet inks for many aspects of ink jet printing, a need still exists for improved inkjet ink formulations of self-dispersed pigment that provide good print quality and good jettability in particular when printed from a thermal ink jet printhead. It is well known to those of ordinarily skill in the art that thermal ink jet printhead has lower tolerance towards the addition of polymer additives on its jettability and reliability compared to piezo ink jet printhead. The present invention satisfies this need by providing compositions having improved optical density, while maintaining other aspects of the ink, dispersion stability, long nozzle life and the like.